1. Field of the Invention
The present invention relates to a solventless silicone pressure-sensitive adhesive composition.
2. Description of the Prior Art
Pressure-sensitive adhesive tapes and pressure-sensitive adhesive labels that use a silicone pressure-sensitive adhesive contain a silicone pressure-sensitive adhesive layer that exhibits excellent levels of heat resistance, cold resistance, weather resistance, electrical insulation and chemical resistance, and they are consequently used under severe conditions where organic pressure-sensitive adhesives such as acrylic pressure-sensitive adhesives or rubber-based pressure-sensitive adhesives would suffer deformation or degradation. Furthermore, because they also exhibit favorable adhesion to all manner of adherends, they are also used for bonding polyolefin resins, silicone resins, fluororesins and moisture-containing surfaces, which are difficult to bond using organic pressure-sensitive adhesives such as acrylic pressure-sensitive adhesives or rubber-based pressure-sensitive adhesives.
For example, during the production, processing or assembly of electronic or electrical componentry, pressure-sensitive adhesive tapes containing a silicone pressure-sensitive adhesive are used. When heat treatments are conducted during these productions steps, the electronic or electrical component or a member thereof often needs to be protected, masked or temporarily secured, either across the entire surface or at a portion of the surface of the component or member. A pressure-sensitive adhesive tape used for these types of applications, or a pressure-sensitive adhesive tape used in the production, securing or bonding of an electronic or electrical component that is exposed to high temperatures preferably employs a silicone pressure-sensitive adhesive that exhibits favorable heat resistance even after heating at temperatures exceeding 250° C. However, modern production and processing steps for electronic or electrical componentry not only often include multiple stages in which heating, cooling, washing and irradiation and the like are conducted repeatedly, but are also requiring increasing levels of precision and complexity. Moreover, in order to prevent dust, processing is often conducted within an enclosed space such as a clean room or a clean booth.
If a pressure-sensitive adhesive tape or pressure-sensitive adhesive label that uses a conventional silicone pressure-sensitive adhesive is bonded to, or used for masking, an adherend, and is subsequently subjected to a heat history at a high temperature of 100 to 250° C., then minute quantities of residual aromatic hydrocarbons within the pressure-sensitive adhesive layer such as toluene, xylene, ethylbenzene and benzene tend to volatilize. These aromatic hydrocarbons are contained within the silicone pressure-sensitive adhesive composition. If a pressure-sensitive adhesive tape that uses a silicone pressure-sensitive adhesive is used within the type of enclosed space described above for the production of an electronic or electrical component, volatile materials such as the above aromatic hydrocarbons are generated, and if ultraviolet irradiation is conducted in a subsequent step, then the ultraviolet radiation is absorbed by the aromatic hydrocarbons, which can cause a deterioration in the efficiency of the irradiation step.
Moreover, recent electronic components continue to become smaller, more detailed and thinner. If protective masking is conducted using pressure-sensitive adhesive tapes that contain widely used pressure-sensitive adhesives, then when the masking tape is peeled away (removed) following completion of a specified treatment, the adhesive strength may sometimes be overly powerful, meaning fine structures on the electronic component that functions as the adherend, or even the electronic component itself, may undergo deformation or damage. Furthermore, when such electronic components are masked, the tape is sometimes bonded over a large surface area. When a masking tape that has been bonded over a large area is peeled away, if the adhesive strength is too powerful, then the peeling operation may be difficult, and the adherend may undergo deformation or damage. Furthermore, even in cases other than those using a pressure-sensitive adhesive tape, such as when a pressure-sensitive adhesive-treated film is bonded to an adherend, if the adhesive strength of the pressure-sensitive adhesive film is overly powerful, then removal may be difficult, and the adherend may undergo deformation or damage. Examples of the adherend include all manner of electronic components, flat panel displays, window glass and wall surfaces, and examples of pressure-sensitive adhesive-treated films include dirt prevention films, scratch prevention films, anti-reflective films, light-shielding films, glass shatter-resistant films, and masking films for decoration or advertising.
Furthermore, in applications such as pressure-sensitive adhesive tapes and pressure-sensitive adhesive labels that are used for construction or interior decorating purposes, the surface of the adherend may either include a silicone resin or fluororesin, or be treated with a silicone resin or fluororesin, in order to provide the surface with favorable water repellency, oil repellency and/or dirt prevention properties. Because acrylic pressure-sensitive adhesives do not exhibit favorable adhesion to these types of surfaces, silicone pressure-sensitive adhesives are often used. In the case of pressure-sensitive adhesive tapes and pressure-sensitive adhesive labels used in living environments, such as pressure-sensitive adhesive papers for bonding wallpaper, pressure-sensitive adhesive papers for bonding decorative sheets for furniture and the like, waterproof and airtight pressure-sensitive adhesive tapes, pressure-sensitive adhesive tapes for interior bonding applications, repair tapes, tapes for securing ornaments, pressure-sensitive adhesive films for shatter-resistant glass, light-shielding pressure-sensitive adhesive films and tapes for securing items inside vehicles, if residual aromatic hydrocarbon solvents are present, and these volatilize and accumulate within the interior airspace, then even if the quantities are extremely small, they can cause chemical substance allergies, sick house syndrome, and chemical substance hypersensitivity.
Furthermore, in applications where bonding is conducted directly to the skin, such as medicinal patches, sticking plasters, surgical tapes, and tapes for sports taping, the existence of residual aromatic hydrocarbon solvents can cause skin irritation.
In order to address these types of problems, a solventless silicone pressure-sensitive adhesive composition has been proposed that comprises an organopolysiloxane having lower alkenyl groups at both terminals of a straight-chain molecular chain, a polyorganosiloxane comprising R3SiO0.5 units and SiO2 units, an organohydrogenpolysiloxane having at least two SiH groups within each molecule, and a platinum-based catalyst (see patent reference 1). However, this composition is prone to increases in the viscosity, can suffer from poor coating properties upon application to substrates, and exhibits a lower level of adhesive strength if the viscosity of the composition is reduced.
Furthermore, pressure-sensitive silicone adhesive compositions with a high solid fraction, in which the main components include a copolymer comprising R3SiO1/2 units and SiO4/2 units, an alkenyl group-terminated polydiorganosiloxane, and a hydride-terminated organohydrogenpolysiloxane are also known (see patent references 2 to 5). However, these compositions are also prone to increases in viscosity, can suffer from poor coating properties upon application to substrates, and exhibit a lower level of adhesive strength if the viscosity of the composition is reduced.
[Patent Reference 1] U.S. Pat. No. 5,110,882
[Patent Reference 2] EP 0506370 A2
[Patent Reference 3] EP 0506371 A2
[Patent Reference 4] EP 0506372 A2
[Patent Reference 5] EP 0581539 A2